If you spend time playing mentally taxing games on your smartphone or computer, will it make you more intelligent? A billion dollar “brain training” industry is premised on the idea that it will. Academic psychologists are divided – the majority view is that by playing brain training games you will only improve at those games, you won’t become smarter. But there are scholars who believe in the wider benefits of computer-based brain training and some reviews support their position, such as the 2015 meta-analysis that combined findings from 20 prior studies to conclude “short-term cognitive training on the order of weeks can result in beneficial effects in important cognitive functions”.

But what if those prior studies supporting brain training were fundamentally flawed by the presence of a powerful placebo effect? That’s the implication of a new study in PNAS that suggests the advertising used to recruit participants into brain training research fosters expectations of mental benefits.

Cyrus Foroughi and his colleagues produced two different recruitment adverts (see image below) to attract participants into a brain training study – one explicitly mentioned that the study was about brain training and mentioned that this training can lead to cognitive enhancements; the other was neutral and simply stated that participants were needed for a study. Nearly all previously published brain training research has used an overt, suggestive style of recruitment advertising.

Nineteen young men and 31 young women signed up in response to the two ads, with no gender or age differences between those who responded to each ad. Next, they completed baseline intelligence tests before spending an hour on a task that features in many commercial brain training programmes – the so-called dual n-back task, which involves listening to one stream of numbers or letters and watching another, and spotting whenever the latest item in one of the streams is a repeat of one presented “n” number of items earlier in that stream. As participants improve, “n” is increased, making the task more difficult. The next day, the participants completed more intelligence tests. They also answered questions about their beliefs in the possibility for people’s intelligence to increase.

The participants who’d responded to the overt, suggestive advert showed gains in intelligence after completing just one hour of brain training – a length of training too short to plausibly have produced any genuine benefit linked to the actual experience of doing the training. In contrast, the participants who responded to the neutral ad showed no intelligence gains. This group difference was despite the fact that the two groups performed just as well on the training task, suggesting no group differences in motivation or ability. Also, the group who’d responded to the suggestive ad reported stronger beliefs in the malleability of intelligence. This could be because people with these beliefs were more likely to respond to the suggestive ad, or because they’d been influenced by the claims of the ad – either way, it shows how the use of unsubtle recruitment advertising could be distorting research in this area.

The researchers said they’d provided “strong evidence that placebo effects from overt and suggestive recruitment can affect cognitive training outcomes”. They added that future brain training research should aim to better reduce or account for these placebo effects, for example avoiding hinting to participants what the goals of the study are, or what outcomes are expected. Their call comes after a group of psychologists warned in 2013 that intervention studies in psychology are afflicted by a “pernicious and pervasive” problem, namely the failure to adequately control for the placebo effect.